Fossilphiles across the United States have been waiting, for what must have seemed like eons, for today, the first-ever National Fossil Day. The event, organized by the U.S. National Park Service, is intended to raise awareness of the importance of preserving fossils and to highlight the vital role that fossils play in scientific education and research. And, of course, it provides fossil enthusiasts an opportunity to indulge openly in all things paleolith.

Dinosaur bones from Alberta, Canada. (AbleStock/Jupiterimages)

Fossils are traces of Earth’s geologic past, and hence they contain secrets about the world’s most ancient life-forms and tell of bygone ages, when creatures of size and form nearly unfathomable to us today ruled the planet. Some of the oldest fossils on Earth are stromatolites, layered deposits consisting primarily of limestone that were formed by the growth of single-celled organisms known as blue-green algae. Stromatolites have been found in rocks more than 3 billion years old. By comparison, the oldest fossil evidence of multicellular life dates to around 2.1 billion years ago. Stromatolites and ancient multicellular fossils have provided valuable information about the evolution of Earth’s early life-forms.

Other fossils, such as ammonoids, serve as index fossils—specimens that are distinct and abundant across a wide geographic region, such that they define a geologic period or environment. Ammonoids are a group of extinct cephalopods, whose fossil remains are typically found in marine rocks that are between about 416 million and 66 million years old. Ammonoids are related to the modern pearly nautilus but are distinguished from the latter by a series of wavy seams that radiate out across the central spiral surface where the organisms’ inner and outer shells join. Because ammonoids have this distinct feature and are found across a broad geographic expanse, they are considered index fossils.

Some of the most familiar fossils, however, are those of dinosaurs. Although people presumably had come across dinosaur fossils many hundreds of years ago, the first dinosaur to be described scientifically was Megalosaurus, fossil remains of which were uncovered in the early 1820s in Britain by William Buckland. Iguanodon, a genus of large herbivorous dinosaurs, was described shortly thereafter, in 1825. In 1842, to formally recognize these giant, extinct creatures, English anatomist Richard Owen introduced the term Dinosauria. Britannica’s dinosaur entry recounts Owen’s observations:

“Owen recognized that these reptiles were far different from other known reptiles of the present and the past for three reasons: they were large yet obviously terrestrial, unlike the aquatic ichthyosaurs and plesiosaurs that were already known; they had five vertebrae in their hips, whereas most known reptiles have only two; and, rather than holding their limbs sprawled out to the side in the manner of lizards, dinosaurs held their limbs under the body in columnar fashion, like elephants and other large mammals.”

In 1856 the first Neanderthal fossil was unearthed. Named for its place of discovery, the Neander Valley in Germany, the specimen played a fundamental role in the development of the field of paleoanthropology. Although there have been relatively few hominin fossil discoveries, paleoanthropologists and physical anthropologists, who investigate the origin, evolution, and diversity of humans, have uncovered several specimens that have proven crucial to current understanding of human evolution. Among these finds are specimens of Australopithecus and Ardipithecus.

In 1992 a molar tooth of the first specimen of Ardipithecus was discovered at Aramis, Ethiopia. In the following years, once more was known about Ardipithecus, the discovery caused some scientists to question existing theories about the evolutionary relationship between humans and Australopithecus, which contains the well-known specimens of Lucy and Selam (Lucy’s child). The evolutionary positioning of Ardipithecus and Australopithecus was discussed in the 2009 Britannica special report “Ardipithecus: A Hominin Ancestor for Lucy?”

Today, new fossils are uncovered around the world on nearly a daily basis. As more fossils are found, new dinosaurs will surely be named and the gaps in scientists’ understanding of the hominin lineage will narrow.